Corrosion in heat exchangers has long been a problem. It is generally recognized that one of the causes of this deterioration is the electrolytic action between the dissimilar metals of the various components. This process, referred to as electrolysis, produces a flow of current between the two unlike metal areas when in the presence of an electrolyte, such as ionized water, which in turn causes corrosion on the more active metal (anode) and forms a protective coating on the less active metal (cathode).
There are a great many factors which control the rate of corrosion in this process, the most important being the relative positions of the metals in the electromotive series. This, of course, determines the electrical potential difference between the materials and thus controls the direction of the current flow. Other factors include water composition, flow velocity, temperature, area relationships of the "electrodes" , flow restriction in the tubes, current density, and polarization which tends to insulate each "electrode" therefore decreasing the potential difference between the metals.
Naturally some metals or alloys withstand this type of corrosion better than others. However, more corrosion resistant materials are generally more expensive. This presents the problem of finding a compromise which will provide a unit with reasonable life at reasonable cost.
To increase life and reduce total cost, a "sacrificial" material is commonly used with heat exchangers prone to produce a current flow. Zinc, being very unstable as evidenced by its position in the galvanic series, is used as a "sacrificial" material in water systems to reduce the deterioration of the more expensive materials used in the highly dense structure of the shell and tube type heat exchangers.
Cost consideration has increased the use of plastic in heat exchangers. For example, U.S. Pat. No. 3,804,161 issued to Leon J. Nowak on Apr. 16, 1974 discloses one such heat exchanger. The utilization of plastic electrically insulating materials has made the problem of eliminating corrosion due to electrolytic action more difficult. For example, in the heat exchanger of Nowak, no current flow will occur from the metal anode tubes through the reinforced plastic heat exchanger to a grounded cathode. Thus, if used with an engine, the electrolytic action will occur between the core and the engine block. The metal of higher potential, the core, will become the anode. The core will tend to go into solution in the electrolyte and corrode. Thus, the life of the heat exchanger will be decreased.
The present invention is directed to overcoming one or more of the problems as set forth above.